1. Field of the Invention
This invention relates to electronic article surveillance systems and more particularly it concerns novel deactivatable and reactivatable markers for use in such systems as well as novel methods of making such markers.
2. Description of the Related Art
It is known to electronically monitor the passage of goods from a protected area by setting up an interrogation zone at an exit from the protected area, attaching special targets or markers to the goods and detecting the presence of the targets or markers when the goods are carried through the interrogation zone. In magnetic type electronic article surveillance systems, a continuous alternating magnetic interrogation field is generated in the interrogation zone; and when a marker is subjected to this field it becomes driven alternately into and out of magnetic saturation and thereby produces characteristic detectable disturbances of the interrogation field. When such a disturbance is detected, the system produces an alarm.
The markers in magnetic type electronic article surveillance systems are usually provided with deactivation elements which comprise magnetizable material of a substantially higher magnetic coercivity than that of the markers. When the deactivation elements are magnetized, they produce magnetic fields which bias the markers into magnetic saturation to such a degree that the magnetic interrogation fields can no longer drive the markers into and out of saturation. Thus the markers are rendered incapable of producing detectable disturbances of the interrogating fields; and the articles to which they are attached may pass through the interrogation zone without activating an alarm. These markers may thereafter be reactivated by demagnetizing their respective deactivation elements. U.S. Pat. Nos. 5,146,204, 5,225,807 and U.S. Pat. No. 4,623,877 describe such markers and electronic article surveillance systems in which such markers are used.
There are two basic types of deactivatable markers which can be used in magnetic electronic article surveillance systems. The first type makes use of a plurality of high coercivity magnetizable elements which are spaced apart and distributed along the length of the marker. These markers can be activated and deactivated by remotely generated magnetic fields, provided that the markers are substantially aligned with these fields the second type of deactivatable marker makes use of a single elongated strip of high coercivity material which extends along the length of the marker. When magnetic elements capable of generating a series of spaced apart magnetic fields are brought into contact with the high coercivity material, they cause a pattern of magnetization to be imposed along the material so that it appears as a series of spaced apart magnetic elements.
In the past, both types of deactivatable marker have been expensive to produce, both from the standpoint of the materials required and from the standpoint of the number of separate manufacturing steps involved in producing the marker. In most cases, the marker and its deactivation element or elements, because of their very different magnetic and mechanical characteristics, had to be separately produced and then assembled. In some instances, for example, as described in U.S. Pat. Nos. 4,950,550 and 5,130,698, it has been proposed that the marker and the deactivation element be formed together and subjected to common drawing and heat treating operations. This however, results in less than optimum processing for the marker or the deactivation element or both. Moreover such process cannot be used for markers which have a series of deactivation elements and therefore they can not be remotely activated and deactivated. Also, as described in U.S. Pat. No. 5,181,021, it has been proposed to form high coercivity deactivation elements by painting onto a marker a coating which comprises high coercivity magnetic powder such as ferric oxide dispersed in a polymer binder. However, the thickness required for such deactivation elements is prohibitively large when such elements are used with markers of thickness greater than 0.001 inch (0.004 mm). U.S. Pat. No. 4,536,229 proposes to separately produce deactivation elements which are cold rolled.
U.S. Pat. No. 4,956,636 proposes a process for manufacturing a deactivatable marker by electroplating a nickel ferrite layer onto a flexible polyester substrate that has been treated by first depositing thereon thin films of chromium and copper to form a "strike layer". Thereafter a hard magnetic layer is made by cutting out strips of magnetic tapes and arranging them on the nickel ferrite layer. This however does not solve the problem of separate manufacture of the hard magnetic layer or of attaching the layer to the marker.